A connector for connecting cells is used for forming a storage battery module for obtaining a specified output voltage by connecting cells in series. FIG. 4 is a sectional view showing a connection constitution of cells in the prior art. FIG. 5(a) and FIG. 5(b) are plan view and side view showing a constitution of a connector in the prior art.
In FIG. 4, a first cell A and a second cell B, respectively, have a metal casing 111 formed cylindrically, and a metal electrode 110 installed at one end, electrically insulated from the metal casing 111. The metal electrode 110 serves also as a positive electrode, and the metal casing 111, as a negative electrode. A bump 114 is set in the center of the metal electrode 110, and a rubber valve 116 is placed in the bump 114 for discharging the gas generated in the cell. By connecting the metal electrode 110 of the first cell A to the metal casing 111 of the second cell B, the first cell A and second cell B are connected in series. This series connection is achieved by welding a connector 115 to the metal electrode 110 and metal casing 111. As shown in FIGS. 5(a) and 5(b), the connector 115 is formed in a circular disk in two-step depth. An opening 117 is formed in the center of the connector 115 in order to evade the bump 114 of the metal electrode 110. The outer surface of a flat portion 118 of the connector 115 is welded to the metal electrode 110 of the first cell A, and the inner surface of a tubular portion 119 is welded to the metal casing 111 of the second cell B. In this way, plural cells are connected in series, and coupled mechanically, so that a battery module of specified output voltage is composed.
In this conventional constitution, welding of the connector 115 and metal electrode 110 is done by spot welding by feeding welding current between the flat portion 118 of the connector 115 and the metal electrode 110. In this case, one welding electrode is fitted to the flat portion 118, and another welding electrode is fitted to the bump 114 of the metal electrode 110. In ordinary spot welding, welding current is passed by pressing two weld zones between the welding electrodes, and pressed portions are fused and welded.
In the connection battery of the prior art, however, since there is no pressing part, most current flows between the welding electrodes as the surface current of the connector 115 and metal electrode 110. Accordingly, when the connector 115 is made of a thick material or has a small specific resistance, welding is difficult. This state causes a similar problem when the tubular portion 119 is welded to the metal casing 111. That is, in the conventional constitution, when the connector 115 is made of a thick material or has a small specific resistance, the electric resistance of the electric connection between cells becomes large, resulting in increased loss due to large current flow. Moreover, the mechanical strength of the connector 115 is weak and has a low welding strength. Therefore, the strength of the mechanical coupling between the cells is also low. Furthermore, since a welding current flows in the recess 114, the rubber valve 116 placed inside of the bump 114 may deteriorate due to heat generation.